Scrutinizing velocity and pressure coupling conditions for LES with downstream RANS calculations

A RANS calculation is connected to an upstream LES via explicit coupling conditions at a pre-defined interface. The role of the interface is to allow for mean flow information to propagate upstream and for fluctuations to leave the LES domain without reflections. To this end, the mean velocity is directly coupled across the domain boundaries whereas the fluctuations at the downstream end of the LES zone are treated using either the so-called enrichment strategy (Quéméré & Sagaut, 2002) or a parameter-free generalization of this method based on a convective condition (von Terzi, Fröhlich & Mary, 2006). For incompressible flows, both techniques require a complementary coupling condition for the pressure or an equivalent variable enforcing continuity. Two distinct techniques are investigated: (i) The instantaneous pressure is computed in a coupled fashion for the union of the LES and RANS domains and (ii) the pressure is completely decoupled and mass conservation across the interface is ensured by an adjustment of the velocities on both sides. The performance of the different methods is scrutinized for turbulent channel flow and the flow over periodic hills. It was found that the convective condition with decoupled pressure fields and an explicit mass flux correction was the most robust technique delivering results of equal or increased quality in comparison to other combinations considered.